1. Field of the Invention
The present invention relates to a novel tin oxide fiber and a process for producing the same. More specifically, the present invention relates to a novel tin oxide fiber having a diameter which is greater than 1 .mu.m or having a length which is greater than 3 mm and having an aspect ratio which is greater than 10, and to a process for producing the same.
2. Prior Art
A tin oxide fiber has been widely known. Being limited by the process of production, however, the tin oxide fibers that have been known so far all have diameters which are smaller than 1 .mu.m. It has been demanded to develop a tin oxide fiber having a large diameter and an increased length to meet a variety of applications. However, a technology that satisfies the above demand has not yet been developed.
In the field of gas sensors, it has been strongly desired to provide the tin oxide in the form of a fiber from the standpoint of improving sensitivity and response time. It has further been attempted to add a carbon fiber or the like in order to give electric conductivity to the high molecular materials. When carbon is used, however, there arise a problem in that the black color of carbon makes it difficult to obtain materials in bright colors. Therefore, it has been attempted to add a metal fiber or the powder of a metal oxide. However, the metal fiber which has a high electric conductivity permits the fiber surfaces to be oxidized or corroded after long periods of time and decreases electric conductivity. Moreover, a conventional metal oxide powder does not have electric conductivity that is as high as that of the metal fibers, and must be added in relatively large amounts to give electric conductivity to the high molecular materials, causing the properties inherent in the high molecular materials to be deteriorated. Attempts have further been made to add the tin oxide having excellent resistance against the chemicals and the heat in the form of a powder. Here, it has been known that the effect for giving electric conductivity increases with an increase in the aspect ratio of the electric conductivity-imparting material. It has therefore been demanded to provide the tin oxide having high electric conductivity in the form of a fiber.
The tin oxide fibers have heretofore been obtained exclusively by the so-called melt-precipitation method according to which the starting material of tin compound is melted at high temperatures in order to precipitate the tin oxide fiber. Japanese Patent Laid-Open Nos. 54997/1985 and 161337/1985 teach tin oxide fibers having a diameter of 0.5 .mu.m and a maximum length of 3 mm obtained by the above melt-precipitation method.
The above methods, however, require a reaction temperature which is higher than 1000.degree. C. and a reaction time of several days. In addition, the obtained fiber has a diameter that is so small that the handling becomes difficult. When used as a composite material, therefore, the functions are not sufficiently exhibited and the applications are restricted. Moreover, the length is so small that paper-like materials cannot be prepared therefrom.